Jet-propelled small-sized boats of the type in which a single person is allowed to ride the boat in a standing posture on the floor deck at the stern while steering the boat for gliding freely over water surface have already been known. Such a small-sized boat has usually mounted thereon a 2-cycle engine having piston valve type cylinders.
In the case of such piston valve type cylinders, any degradation of fluid-tightness between the slide surfaces of the cylinders and piston rings caused as by deterioration of piston rings would immediately lead to a decrease in the engine output, adversely affecting the engine start characteristic and fuel cost.